User equipment (UE), also known as mobile stations, wireless terminals and/or mobile terminals are enabled to communicate wirelessly in a wireless communication system, sometimes also referred to as a cellular radio system. The communication may be made e.g. between two user equipment units, between a user equipment and a regular telephone and/or between a user equipment and a server via a Radio Access Network (RAN) and possibly one or more core networks.
The user equipment units may further be referred to as mobile telephones, cellular telephones, laptops with wireless capability. The user equipment units in the present context may be portable and enabled to communicate voice and/or data, via the radio access network, with another entity, such as a network node, for example.
The wireless communication system covers a geographical area which is divided into cell areas, with each cell area being served by a network node, or base station e.g. a Radio Base Station (RBS), which in some networks may be referred to as “eNB”, “eNodeB”, “NodeB” or “B node”, depending on the technology and terminology used. The network nodes may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the network node/base station at a base station site. One base station, situated on the base station site, may serve one or several cells. The network nodes communicate over the air interface operating on radio frequencies with the user equipment units within range of the respective network node.
In some radio access networks, several network nodes may be connected, e.g. by landlines or microwave, to a Radio Network Controller (RNC) e.g. in Universal Mobile Telecommunications System (UMTS). The RNC, also sometimes termed a Base Station Controller (BSC) e.g. in GSM, may supervise and coordinate various activities of the plural network nodes connected thereto. GSM is an abbreviation for Global System for Mobile Communications (originally: Groupe Spécial Mobile).
UMTS is a third generation mobile communication system, which evolved from the GSM, and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units.
The 3rd Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies, for example by developing Long Term Evolution (LTE) and the Evolved Universal Terrestrial Radio Access Network (E-UTRAN).
LTE is a technology for realizing high-speed packet-based communication that may reach high data rates both in the downlink and in the uplink. In LTE, network nodes, or base stations, which may be referred to as evolved-NodeBs, eNodeBs or even eNBs, may be connected to a gateway e.g. a radio access gateway, which in turn may be connected to one or more core networks.
In the present context, the expressions downlink, downstream link or forward link may be used for the transmission path from the network node to the user equipment. The expression uplink, upstream link or reverse link may be used for the transmission path in the opposite direction i.e. from the user equipment to the network node.
One important focus area in E-UTRAN standardisation work is to ensure that the new network is simple to deploy and cost efficient to operate. This disclosure concerns a method for random access interference avoidance, typically between cells of different sizes, for example between a macro cell on the one hand and a micro/pico/femto or even a relay on the other. This situation may further be referred to as a heterogeneous network, sometimes referred to as a hetnet.
In LTE the downlink is based on Orthogonal Frequency Division Multiplexing (OFDM) while the uplink is based on a single carrier modulation method known as Discrete Fourier Transform Spread OFDM (DFT-S-OFDM).
In this document is embodiments related to the LTE random access disclosed. Therefore, the random access procedure for LTE as it is currently defined by 3GPP is briefly summarised. Subsequently, the LTE random access concept will be briefly presented.
During initial access, the user equipment seeks access to the network in order to register and commence services. The Random Access (RA) serves as an uplink control procedure to enable the user equipment to access the network. Since the initial access attempt cannot be scheduled by the network, the random access procedure is by definition contention based. Collisions may occur and an appropriate contention-resolution scheme needs to be implemented. Including user data on the contention-based uplink is not spectrally efficient due to the need for guard periods and retransmissions. Therefore, it has been decided to separate the transmission of the random access burst (preamble), whose purpose is to obtain uplink synchronisation, from the transmission of user data.
The random access procedure serves two main purposes: to let the user equipment align its uplink timing to that expected by the network node, or eNodeB, in order to minimize interfering with other uplink transmissions made by other user equipment. Uplink time alignment is a requirement in E-UTRAN before data transmissions may commence.
In addition, the random access procedure also provides a means for the user equipment to notify the network of its presence and enables the network node to give the user equipment initial access to the system.
Furthermore, in addition to the usage during initial access, the random access may also be used when the user equipment has lost the uplink synchronisation or when the user equipment is in an idle or a low-power mode.
The basic random access procedure comprises a four-phase procedure, wherein the first phase comprises transmission of random access preamble, allowing the network node to estimate the transmission timing of the user equipment. Uplink synchronisation is necessary as the user equipment otherwise cannot transmit any uplink data. In the second phase, the network transmits a timing advance command to correct the uplink timing, based on the timing of arrival measurement in the first phase. In addition to establishing uplink synchronisation, the second phase also assigns uplink resources and temporary identifier to the user equipment to be used in the third phase in the random access procedure. The third phase comprises signalling from the user equipment to the network using the UL-SCH similar to normal scheduled data. A primary function of this message is to uniquely identify the user equipment. The exact content of this signalling depends on the state of the user equipment, e.g., whether it is previously known to the network or not. The last, fourth phase, is responsible for contention resolution in case multiple user equipments tried to access the system on the same resource.
For cases where the network knows, in advance, that a particular user equipment will perform a random access procedure to acquire uplink synchronisation, a contention-free variety of the random access procedure has been agreed. This effectively makes it possible to skip the Contention Resolution process of the last two phases for important cases such as arrival to target cell at handover and arrival of downlink data.
For the event of Random Access overload, a random access back-off procedure is supported. This procedure prevents immediate new Random Access attempts which would only worsen a collision situation.
In case of an overload, the network node signals through the random access response message a back-off indicator TB. The user equipment that does not receive a random access response message that includes the transmitted preamble will wait a time which is uniformly distributed between 0 and TB before attempting random access gain.
The idle mode cell selection and reselection procedure in LTE is based on both stored information, information acquired from broadcasted system information and evaluations of radio frequency measurements by the user equipment.
The cell selection evaluation process is based on a criterion S, which is fulfilled when:Srxlev>0 AND Squal>0where:Srxlev=Qrxlevmeas−(Qrxlevmin+Qrxlevminoffset)−PcompensationSqual=Qqualmeas−(Qqualmin+Qqualminoffset)Where:
SrxlevCell selection RX level value (dB)SqualCell selection quality value (dB)QrxlevmeasMeasured cell RX level value (RSRP)QqualmeasMeasured cell quality value (RSRQ)QrxlevminMinimum required RX level in the cell (dBm)QqualminMinimum required quality level in the cell (dB)QrxlevminoffsetOffset to the signalled Qrxlevmin taken into account in theSrxlev evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in aVPLMNQqualminoffsetOffset to the signalled Qqualmin taken into account in theSqual evaluation as a result of a periodic search for ahigher priority PLMN while camped normally in aVPLMNPcompensationmax(PEMAX − PPowerClass, 0) (dB)PEMAXMaximum TX power level an UE may use whentransmitting on the uplink in the cell (dBm) defined asPEMAX in [TS 36.101]PPowerClassMaximum RF output power of the UE (dBm) accordingto the UE power class as defined in [TS 36.101]
The signalled values Qrxlevminoffset and Qqualminoffset are only applied when a cell is evaluated for cell selection as a result of a periodic search for a higher priority PLMN while camped normally in a VPLMN. During this periodic search for higher priority PLMN the user equipment may check the S criteria of a cell using parameter values stored from a different cell of this higher priority PLMN.
Self-backhauling relays are considered for LTE Advanced. LTE-Advanced extends LTE Rel-8 with support for relaying as a tool to improve e.g. the coverage of high data rates, temporary network deployment, the cell-edge throughput and/or to provide coverage in new areas.
The connection such as e.g. Un between the network node and the relay node may be inband, in which case the link from the network node to the relay node share the same band with direct network node to user equipment links within the donor cell. However, the Un connection between the network node and the relay node may be outband, in which case the link from the network node to the relay node does not operate in the same band as direct network node to user equipment links within the donor cell. At least “Type 1” relay nodes are supported by LTE-Advanced. A “Type 1” relay node is an inband relay node characterised by that it control cells, each of which appears to a user equipment as a separate cell distinct from the donor cell. Also, the cell controlled by the inband relay node have their own Physical Cell ID, i.e. a fingerprint used by user equipment to identify the cell, and transmit their own synchronization channels, reference symbols etc. In the context of single-cell operation, the user equipment receives scheduling information and data transmission feedback directly from the relay node and sends its control channels to the relay node. In addition, the inband relay node appears as a network node to legacy user equipment, i.e. it is backwards compatible. The inband relay node may further be inactive at times for example to save energy. To a large extent, the relay nodes may be perceived as any network node. For example, the connections X2 and S1 between relay node and other network nodes may be established, e.g. partly over Un.
Furthermore, the relay node is handled to a large extent as any user equipment served by the serving network node. For example, when the relay node is installed, it attaches to the network via the user equipment attach procedure, and first when Radio Resource Control (RRC) connectivity is established, the serving network node is informed by the core network that the user equipment in fact is a relay node.
In a cellular network there may be areas with high traffic, i.e. high concentration of user equipment, at least during some time of the day. In those areas it may be desired to deploy additional capacity to keep the user satisfaction. The added capacity may then be in the form of additional macro base station, or to deploy nodes with lower output power and thus covering a smaller area in order to concentrate the capacity boost on a smaller area.
There may also be areas with bad coverage where there is a need for coverage extension, and again one way to do that may be to deploy a node with low output power to concentrate the coverage boost in a small area.
One argument for choosing nodes with lower output power in the above cases may be that the impact on the macro network may be reduced, or even minimized, as it is a smaller area where the macro network may experience interference.
Currently there is a strong drive in the industry in the direction towards the use of low power nodes. The different terms used for this type of network deployments are Heterogeneous networks, multilayer networks or shortly HetNets.
Thus a macro base station, as the network node may be referred to, provides a wide area coverage, also called macro cell. Low power nodes are deployed to provide small area capacity/coverage. Some examples of such low power nodes may be pico base stations, relays, home base stations and/or femto cells. A pico base station may either be similar to a macro network node but typically with more limited coverage for example featuring a lower max transmission power, or a remote radio unit connected to a main unit. A common term for the pico/relay/femto cells is underlay cells, served by underlay nodes. The cells may either be open access, or providing access only to a Closed Subscription Group (CSG).
The femto base stations, or any underlay nodes, may be connected to the MME via a femto gateway, providing any or all of the following features: Control plane signalling to/from the MME via the interface S1_MME; user plane data to/from the packet gateway via the interface S1_U; and/or OaM interface based on Broadband Forum TR-069.
Furthermore, the underlay nodes may be interconnected as well as connected to other network nodes via X2 interfaces.
The interaction between network nodes, or macro eNodeBs, on the one hand and the underlay nodes on the other hand may be one of the following: No interaction at all. This may be the case when X2 is not available, e.g. for close access femto cells. The interaction may further be a loose interaction. This may be the case when X2 is available. A third option for the interaction between network nodes and underlay nodes may be tight interaction. This may be the case when underlay node is a remote unit, typically connected to the macro network node via a low latency connection.
The underlay nodes may typically operate at lower reference (pilot/perch) signal powers compared to the macro network nodes. This means that if the cell selections as well as mobility decisions are based on received reference signal strengths, the downlink cell border is closed to the underlay node than to the macro network node. If the uplink sensitivity for all cells is similar, or if the difference in uplink sensitivity is not equivalent to the difference in reference (pilot/perch) signal powers, then the uplink cell border will be different from the downlink cell border.
This means that a user equipment served by the macro network node may have the best uplink to an underlay node, causing extensive uplink interference even without having detected the underlay reference signal.
One means to relieve this situation is to consider an underlay cell range expansions by considering offsets in the cell selection and/or mobility decisions. This is referred to as Cell Range Expansion (CRE). Thereby, potentially interfering user equipment served by the macro network nodes are further away from the underlay node and thereby inducing less interference. However, this also means that some user equipment served by the underlay node may be critically interfered by the macro network node in the downlink.
One way to manage this interference is via announced almost blank subframes in the macro network node, where the macro network node avoids scheduling users in selected subframes, so that the underlay node may schedule user equipment in these low interference subframes. Similarly, the macro network node may schedule user equipment in the selected subframes, but at a reduced power level. The macro node may or may not also consider transmitting downlink control information in selected subframes, possibly at reduced power levels.
Cell range expansion may be considered in both idle and connected mode. The main difference between the two situations is the signalling means and the knowledge about the user equipment. In idle mode, there is a need to support random access and paging—procedures that involve user equipment that the network has no or very little information about considering the radio propagation and interference situation. Moreover, signalling is more limited and restricted to broadcast or stored information from previous connection sessions.
On the contrary, in connected mode, there may be quite good information about the user equipment with respect to its radio situation. Furthermore, dedicated signalling means to specific user equipment are available.
There are several issues with supporting cell range expansion and adequate interference avoidance with almost blank subframes for idle mode user equipments as discussed above. However, if not considering cell range expansion for idle mode user equipment, then they will immediate perform handover to the underlay node from the macro network node when establishing an RRC connection if cell range expansion is employed in connected mode. This may however generate many handovers.